# Nagell–Lutz theorem

In mathematics, the Nagell–Lutz theorem is a result in the diophantine geometry of elliptic curves, which describes rational torsion points on elliptic curves over the integers.

## Definition of the terms

Suppose that the equation

${\displaystyle y^{2}=x^{3}+ax^{2}+bx+c\ }$

defines a non-singular cubic curve with integer coefficients a, b, c, and let D be the discriminant of the cubic polynomial on the right side:

${\displaystyle D=-4a^{3}c+a^{2}b^{2}+18abc-4b^{3}-27c^{2}.\ }$

## Statement of the theorem

If P = (x,y) is a rational point of finite order on C, for the elliptic curve group law, then:

• 1) x and y are integers
• 2) either y = 0, in which case P has order two, or else y divides D, which immediately implies that y2 divides D.

## Generalizations

The Nagell–Lutz theorem generalizes to arbitrary number fields and more general cubic equations.[1] For curves over the rationals, the generalization says that, for a nonsingular cubic curve whose Weierstrass form

${\displaystyle y^{2}+a_{1}xy+a_{3}y=x^{3}+a_{2}x^{2}+a_{4}x+a_{6}\ }$

has integer coefficients, any rational point P=(x,y) of finite order must have integer coordinates, or else have order 2 and coordinates of the form x=m/4, y=n/8, for m and n integers.

## History

The result is named for its two independent discoverers, the Norwegian Trygve Nagell (1895–1988) who published it in 1935, and Élisabeth Lutz (1937).